Seabed located storage

ABSTRACT

Seabed located storage for crude oil or other fluid, distinguished in that it is comprising a storage section in form of an oil and waterproof cloth formed as a flexible balloon that can be filled with, store and emptied for a storage fluid, a structure section formed as an external casing over the storage section, which structure section is closed in the upper part such that an upward close volume of size at least corresponding to the volume of the storage section is formed, but with openings to the surroundings in the lower part, an anchoring section formed as a substructure between the structure section and the seabed, with means for anchoring to or stable placement on the seabed, and a transfer section comprising pipes and valves for loading and unloading of a storage fluid, arranged in substance exterior to the upper part of the storage.

FIELD OF THE INVENTION

The present invention regards storage at sea of crude oil or otherfluid. The invention regards in particular seabed located storage ofstabilized crude oil.

The invention is in particular useful in connection with marginal oilfields for which the expenses for a separate pipeline to shore or toanother installation cannot be justified or oil fields in their endperiod of operation, for which costs to investments and maintenance areto be kept at low level. The invention is also beneficial with respectto oil fields for which it is desirable with continuous production anddelivery of produced crude oil, and for which it is desirable with astockpile, for example during temporary shut down of an existing pipesystem for delivery of the produced crude oil.

PRIOR ART AND BACKGROUND OF THE INVENTION

Storage of crude oil at sea at present usually takes place at seasurface, on floating storage vessels, in loading buoys with storagecapacity, or on the production unit itself in form of a platform or aship.

Floating production platforms with delivery of the crude oil by use of atanker is dependent of having storage capacity on the platform or nearbyin form of a storage ship or loading buoy with storage capacity, fromwhere the oil can be transported further for example to a land terminal.

Platforms fixed to the seabed have in some embodiments a large fundamentthat also is comprising storage for produced oil.

The storage for the produced oil is in many respects functioning asballast, and out of consideration to stability and mechanical loads theoil may have to be replaced with ballast water upon unloading to atanker. Usually seawater is let into a tank simultaneously as oil ispumped out. By filling of the storage seawater is likewise displaced orpumped out after having passed an oil-water-meter that controls the oilcontents of the water before it is discharged. Maximum allowed oilcontents of the water to be discharged varies, but is most commonly atpresent 40 ppm. In connection with storage in the above-mentioned way anundesirable emulsion layer will be formed in the oil load, includingoil, water and chemicals. Chemicals are added to keep oil and waterbetter separated. Further, the ingress of relatively small amounts ofseawater into the oil load results in undesired salt contents andthereby significantly reduced value of the load. With awater-oil-storage of the above-mentioned type a usual problem isundesired bacterial growth (termed SRB), which in combination with waterresults in sulphuric acid. It is known that said bacterial growth hasresulted in severe problems for tank and pipe systems both on platformsand buoy loading tankers.

The above-mentioned emulsion layer has tendency to gain thickness, andthe storage capacity is therefore reduced over time. The emulsion layercan be pumped over to a tanker and transported to a land terminal orrefinery to separate water, oil and chemicals and render hazardouscomponents harmless. Said separation and rendering harmless of hazardouscomponents are very expensive and only very few refineries or othersundertake such jobs.

A demand exists for providing storage of crude oil or other fluidswithout the above-mentioned problems. Further, a demand exists forproviding a flexible storage for crude oil in such a way that storageunits can be transported to where a demand for storage exists, adaptedto the actual demand, and allowing production or continued productionfrom marginal fields. A demand also exists for storage without dangerfor collision with surface vessels, ice or drifting objects.

SUMMARY OF THE INVENTION

With the present invention a subsea located storage is provided forstorage of crude oil or other fluid, distinguished in that it iscomprising

a storage section in form of an oil and waterproof cloth formed as aflexible balloon that can be filled with, store and emptied for astorage fluid,

a structure section formed as an external casing over the storagesection, which structure section is closed in the upper part such thatan upward close volume of size at least corresponding with the storagesection volume is formed, but with openings to the surroundings in thelower part,

an anchoring section formed as a substructure between the structuresection and the seabed, with means for anchoring to or stable placementon the seabed, and

a transfer section comprising pipes and valves for loading and unloadingof the storage fluid, arranged in substance exterior to the upper partof the storage.

The objective of the invention is met by using a storage section in formof a cloth that is fabricated to a large flexible balloon that isfastened into a structure made up of ferroconcrete, steel or anotherfeasible material, where the upper part of the structure is closed tothe surroundings such that the oil of the storage section can be keptconfined if leakage occurs and which structure in the lower part is openand attached to an anchoring section that is keeping the storage fixedto the seabed. Loading and unloading takes place via pipes from aproduction platform or similar, and risers are arranged from the storageto a buoy loading vessel or similar for disposal of the oil.

All valves, instruments and any pumps in connection to the storage willmost preferably be possible to operate remotely from the productionplatform via hydroacoustic signals, optionally via cables.

An oleometer and a hydroacoustic transponder are most preferablyarranged on the storage to give an alarm to the production platform orthe ship if oil leakage occurs, such that corrective measuresimmediately can commence. The oleometer is preferably arranged withconnection to a pipe from the upper part of the structure section'sclosed volume to the unloading pipe from the storage. The pipe withconnected oleometer preferably includes a remotely controllable pump topump oil leaked out into an unloading pipeline. Leaked out oil canoptionally be pumped up to the vessel via a separate pipeline.

In top of the structure section connections are most preferablyarranged, which can be opened to replace the storage section, forexample at leakage, and in addition connections for replacement ofmodules of the transfer section, for example for maintenance orreplacement of valves.

The storage preferably comprises valves that automatically are closed ifloading lines, unloading lines or the storage section and transfersection are disconnected from the IS storage.

The storage according to the invention preferably comprisesinstrumentation for monitoring the filling volume of the storagesection, readable from the surface. Ultrasound based instrumentation isconsidered feasible.

One or more pumps are preferably integrated into the storage, to easeunloading of oil. However, pumps are not necessarily required, and aretherefore not obligatory. The lower specific gravity (15% lower specificgravity) of the oil will together with the capillary effect give adegree of flow or drift of oil upwards. Pumps on the loading vessel canpreferably be used, and optionally pumps arranged in the unloading linea little below the sea level, to pump oil up to the storage tanks of thevessel.

With the storage according to the present invention full separation ofoil and water is achieved, such that formation of an emulsion layer isavoided and the danger of bacterial growth (SRB) is eliminated. Further,a large degree of flexibility is achieved, which means that the storagecan be connected to any type of oil production plant, it can easily beplaced and moved to new fields, and the storage is independent of waterdepths within the depths for which oil production presently and inforeseeable future takes place. The water pressure around the storagehelps to give a uniform filling and effective unloading of the storage.The storage can relatively easy be taken up and materials berecirculated to other purposes after expiry of the life period. Thestorage according to the invention can also be used for gas storage inaddition to storage of different liquids, with adaptation of theconstruction to handle the increased buoyancy.

DRAWINGS

The storage according to the invention is illustrated by drawings, ofwhich:

FIG. 1 illustrates an embodiment of the storage according to theinvention.

FIG. 2 illustrates the storage according to the invention, withindication of the means to replace the storage section.

FIG. 3 illustrates an assembly of several storages according to theinvention.

FIG. 4 illustrates a typical arrangement with use of the storageaccording to the invention.

DETAILED DESCRIPTION

Reference is first made to FIG. 1 illustrating the storage according tothe invention. More specific a storage 1 of oil- and waterproof flexiblematerial is illustrated. Further, a structure section 2 is illustrated,which can be made from ferroconcrete, steel or other feasibleconstruction material. Openings 3 for free passage of seawater areprovided in the lower part of the structure section. Arranged is aremotely controllable valve 4 between the storage section and theunloading line. In connection to a pipe that is extending between theupper part of the structure section and the unloading pipe it isarranged a remotely controllable valve 7, an oleometer 5 forregistration of oil contents between the storage section and the upperpart of the structure section, in addition to a transponder 6 for alarmto the platform or optionally to a buoy loading ship at oil leakage.With said pipe leaked out oil can be brought into the unloading pipewith a dedicated pump (not illustrated). Another remotely controllablevalve 8 is arranged between the platform and the storage, at the top ofthe storage. A pipe socket 9 conducts loading oil into the storagesection. Pad eyes 10 are arranged for easy connection of liftingequipment in connection with replacement of the storage section. In thelower part of the storage steel skirt 11 is arranged for stabilizationof the storage against the seabed, and anchoring 12 in form of suctionanchors, piles or similar for fastening to the seabed. A pipe 13, forexample a flexible pipe, conducts oil from the production platform tothe storage. Another pipe 14, for example a flexible pipe, is arrangedto unload oil from the storage to a buoy loading tanker or similar.

On FIG. 2 it is indicated how replacement of the storage section can beundertaken. It is the intension that all equipment that can have demandfor maintenance or replacement, such as valves, instruments, pumps andthe storage section, can be disconnected from the storage and brought tothe surface, either together or as modules of equipment, preferably byuse of ROV-operatable bolts, connections and fixing points.

FIG. 3 illustrates an assembly of several storages according to thepresent invention, illustrating from left to right a nearly emptystorage section, a full storage section and a storage section withleakage. The transfer section comprises a manifold connected to theplatform and also to the loading system. The transfer section per sewith valves, pipes and oleometer has a little different constructionthan illustrated on FIG. 1, however, the functionality is similar.

The storage according to the invention, or assemblies thereof, can beconnected to one or more similar or different sources for load and oneor more similar or different receivers for load, including subseaproduction plants and subsea pipeline installations.

On FIG. 4 a typical arrangement is illustrated, for which the storageaccording to the present invention is illustrated in a broader context,located on the seabed connected to a production platform for loading andconnected to a tanker for unloading.

The specific construction of the different parts of the storageaccording to the invention can be varied, provided that thefunctionality and distinguishing features according to the invention aremaintained. The construction materials can be chosen freely underconsideration to strength, weight, durability and price. It isconsidered that the stress during transport and deployment/lifting willbe essential for dimensioning. The form of the storage section and theupper part of the struction section can preferably be symmetrical aboutthe vertical axis, taking into account stability, strength and easyfilling and unloading. Taking into account the fabrication it can bemore preferable with a more square form around the vertical axis, suchas for the outer surface of the structure section illustrated on thedrawings.

The structure section can preferably be manufactured as a modifiedembodiment of Unibag from Enviro Team, Oslo, Norway. Unibag is used forstorage and transport of water, oil and chemicals. The standardembodiment of the Unibag is prepared from a fabric with the term Protex092, which is oil- and waterproof and easily weldable with highfrequency equipment. More specific the fabric is a woven polyestercoated on both sides with a chlorinated cross bound ethylene basedinter-polymer alloy. The coating is rubber like but vulcanizing is notrequired, and softeners are not leaching out as for normal PVC-coatedmaterials. Preferable modifications are comprising, according to demand,increased size, reinforcements and inside or outside inserts, animpermeable jacket or liner of LDPE, PA, PVDF or similar for long termfunctionality.

The storage volume is limited with respect to strength and ability forhandling. Storage volumes up to about 35 000 m³ are consideredpreferable. This corresponds to a radius of about 20 m if the storagesection is a sphere. The storage section can, however, be a half sphere,be pear-shaped, be rectangular or have different form.

In a preferable embodiment of the invention two storage sections, eachof about 35 000 m³, are assembled within a common structure section andanchoring section. Dimensions thereby become 35 m×35 m×102 m and with astructure section in steel the weight becomes about 5000 tons. Thestructure section is thereby considered to comprise 12 mm plates withL300 stiffeners and portal frames (T 2500×600×20×30) each third meter.If the steel is replaced with concrete the weight becomes about 30 000tons. It is of course possible to assemble several storage sections inone structure section, or to assemble storage modules according toactual demand.

Most preferable suction anchors are used if the seabed conditions areacceptable, else piles are used for anchoring.

The anchoring section can preferably be an integrated part of thestructure section. Thereby means for anchoring or stable placement onthe seabed are integrated in the structure section, without furthercomponents being a part of the anchoring section.

The storage according to the invention can be fabricated at a yard andtransported out with a vessel. Transport can be undertaken by filling asufficient amount of air in the storage section such that the storage oran assembly of several storages can be stably kept floating undertowing, with bottom weights to ensure stability.

Deployment of the storage preferably takes place by use of a cranevessel or in at least a crane means. An appropriate volume of air isdischarged through hatches, pipes or other outlets such that a weakpositive buoyancy is achieved, while the storage or an assembly ofseveral storages are handled stably of the crane. By beginningdeployment air is discharged in a controlled way until a weak negativebuoyancy is achieved. As the is storage is deployed into the sea the airwill be compressed and the buoyancy will become more and more negative,which has to be handled entirely by the crane vessel or similar.

Lifting of the storage or an assembly of several storages is alsoundertaken by use of a crane vessel or similar. If the depth is not toolarge the storage section can be filled with air to ease lifting,however, not to such extent that uncontrolled positive buoyancy isresulting close to the sea surface.

By using buoyancy elements that are kept at the surface the deploymentand lifting can be undertaken without the use of a crane vessel.

1. (canceled)
 2. The storage arrangement according to claim 1, furthercomprising an oleometer and an acoustic transponder connected forsensing signaling oil leakage.
 3. The storage arrangement according toclaim 2, further comprising: a leakage recovery line located at an upperpart of the inner volume of the structural section and leading to theunloading line from the storage member, and recovery pump to pump oilleaked from the storage member, via the recovery line, into theunloading line.
 4. The storage arrangement according to claim 13,further comprising valves a loading line valve and an unloading linevalve that close automatically if any of the loading line, unloadingline of storage member and transfer section are disconnected from thestorage arrangement.
 5. The storage arrangement according to claim 13,further comprising surface-readable instrumentation for measuring afilling volume of the storage member.
 6. The storage arrangementaccording to claim 13, further comprising at least one integratedauxiliary unloading pump.
 7. The storage arrangement according to claim4, further comprising: a remotely operable recovery pump to pump oilleaked from the storage member, via a recovery conduit, into theunloading conduit; at least one integrated auxiliary unloading pump; andinstrumentation for measuring a filling volume of the storage member; inwhich the loading line and unloading line valves, the recovery andauxiliary unloading pumps and the instrumentation are operable from thesurface.
 8. The storage arrangement according to claim 13, in which themaximum storage volume of the storage member is about 35 000 m³ orsmaller.
 9. The storage arrangement according to claim 4, in which: thestructural section has a top part to which the storage member isconnected; and the top part is removable from the storage arrangementwhile the storage arrangement is submerged, the storage member therebybeing replaceable and serviceable even when the rest of the storagearrangement is submerged
 10. The storage arrangement according to claim13, that in which the storage member is fabricated from woven polyestercoated on each side with a chlorinated cross-bound ethylene basedinterpolymer alloy, with reinforcements and an internal impermeableliner of LDPE.
 11. The storage arrangement according to claim 13, thatin which a plurality of storage members and respective structuralsections, are included within a common main structure.
 12. The storagearrangement according to 13, in which the anchoring section is anintegrated part of the structural section.
 13. A storage arrangement forstoring a fluid such as crude oil comprising: at least one storagemember formed as a flexible balloon of an oil- and waterproof materialhaving a maximum storage volume into which the fluid is pumped forstorage and out of which the fluid is removed for transfer; an upwardlyclosed structural section that has an upwardly delimited inner volumethat is at least as great as the maximum storage volume and within whichthe storage member is located; and lower openings; an anchoring sectionanchoring the structural section to a seabed; a transfer sectionincluding: a loading line leading the fluid into the storage member viaa loading valve; and an unloading line leading fluid from the storagemember via an unloading valve.
 14. A storage arrangement for storing afluid such as crude oil comprising: at least one storage member formedas a flexible balloon of an oil- and waterproof material having amaximum storage volume into which the fluid is pumped for storage andout of which the fluid is removed for transfer; an upwardly closedstructural section that has an upwardly delimited inner volume that isat least as great as the maximum storage volume and within which thestorage member is located; and lower openings; an anchoring sectionanchoring the structural section to a seabed; a transfer sectionincluding a loading line leading the fluid into the storage member via aloading valve; an unloading line leading fluid from the storage membervia an unloading valve; a loading line valve and an unloading line valvethat close automatically if any of the loading line, unloading linestorage member and transfer section are disconnected from the storagearrangement; an oleometer and an acoustic transponder provided forsensing and signaling oil leakage; a leakage recovery line located at anupper part of the inner volume of the structural section and leading tothe unloading line from the storage member; a recovery pump to pump oilleaked from the storage member, via the recovery line, into theunloading line; and surface-readable instrumentation for measuring afilling volume of the storage member.